1. Field of the Invention
The present invention relates to a unit cell for use in a bipolar, filter press type, aqueous alkali metal chloride solution electrolytic cell. More particularly, the present invention is concerned with an improvement in and relating to a unit cell for use in a bipolar, filter press type, aqueous alkali metal chloride solution electrolytic cell comprising a plurality of unit cells which are arranged in series through a cation exchange membrane disposed between respective adjacent unit cells, each unit cell comprising: an anode-side pan-shaped body having an anode compartment and an anode-side gas-liquid separation chamber which extends over the entire length of the upper side of the anode compartment, and a cathode-side pan-shaped body having a cathode compartment and a cathode-side gas-liquid separation chamber which extends over the entire length of the upper side of the cathode compartment, wherein the anode-side pan-shaped body and the cathode-side pan-shaped body are disposed back to back, wherein the anode-side and cathode-side gas-liquid separation chambers have perforated bottom walls separating the anode-side and cathode-side gas-liquid separation chambers from the anode compartment and the cathode compartment, respectively. The improvement comprises a bubble removing partition wall which is disposed at least in the anode-side gas-liquid separation chamber of the anode-side and cathode-side gas-liquid separation chambers and which extends upwardly of the perforated bottom wall of the gas-liquid separation chamber, wherein the bubble removing partition wall extends along the entire length of the gas-liquid separation chamber to partition the gas-liquid separation chamber into a first passage A formed on the bottom wall in a perforated area thereof and a second passage B which is formed on the bottom wall in a non-perforated area thereof and which communicates with a gas and liquid outlet nozzle, and wherein the bubble removing partition wall has an apertured segment and, the apertures of the apertured segment of the bubble removing partition wall are positioned at least 10 mm above the inside surface of the bottom wall of the gas-liquid separation chamber.
The unit cell of the present invention is advantageous in that a gas and an electrolytic solution can be discharged in a condition wherein the gas and the electrolytic solution are substantially completely separated from each other. Therefore, the electrolytic cell which employs the unit cell of the present invention has an advantage in that, even when an electrolysis is performed at high current density, the occurrence of a breakage of an ion exchange membrane due to vibrations in the electrolytic cell can be suppressed.
2. Prior Art
In general, for stably performing the electrolysis of an alkali metal chloride to enable low-cost production of chlorine, hydrogen and an alkali metal hydroxide, it is required that the cost of equipment be low, that the electrolytic voltage be low, that vibrations or the like in the electrolytic cell do not cause a breakage of an ion exchange membrane, and that the concentration distribution of an electrolytic solution in an electrode compartment be narrow, thereby causing the voltage and the current efficiency of an ion exchange membrane to be stable for a prolonged period of time, and so on.
In recent years, in accordance wit the above-mentioned requirements, remarkable progress has been made in the technology for the electrolysis of an alkali metal chloride using an ion exchange membrane (i.e., the technology for the ion exchange membrane electrolysis). The improvements are especially remarkable in the performances of the ion exchange membranes, electrodes and electrolytic cells. At the time the ion exchange membrane electrolysis was introduced for the first time, the electricity consumption of the ion exchange membrane electrolysis performed at a current density of 30 A/dm2 was as large as 2,600 kW per ton of NaOH produced. However, as a result of the above-mentioned great progress in the art in recent years, the electricity consumption of the ion exchange membrane electrolysis performed at a current density of 30 A/dm2 has been reduced to about 2,000 kW or less per ton of NaOH produced. On the other hand, it has recently been strongly desired that the size of the equipment for performing the electrolysis is increased, energy is saved, and efficiency is increased. In addition, it has also been desired for the electrolysis to be able to be performed at a current density as high as 50 A/dm2 or more, which is far higher than the above-mentioned current density 30 A/dm2 which was the possible maximum value at the time of the introduction of the ion exchange membrane electrolysis.
However, when the electrolysis is performed at high current density, the amount of a gas formed is increased, causing an increase in the pressure fluctuations in the electrolytic cell, so that vibrations are likely to be generated in the electrolytic cell. When the electrolysis at high current density is performed for a long time, there has conventionally been posed a problem in that the vibrations in the electrolytic cell can cause a breakage of an ion exchange membrane.
Especially, in the anode compartment of the unit cell of an alkali metal chloride electrolytic cell, gas bubbles have a great influence. For example, when the electrolysis is performed under electrolysis conditions wherein the current density is 40 A/dm2, the reaction pressure is 0.1 MPa, and the reaction temperature is 90 xc2x0 C., the upper portion of the anode compartment is filled with gas bubbles, so that the electrolytic solution in the upper portion of the anode compartment is likely to have portions containing gas bubbles in an amount as large as 80 % by volume or more. The ratio of such high bubble content portions in the electrolytic solution tends to be increased in accordance with an increase in the current density.
Such portion of the electrolytic solution wherein the gas/liquid ratio is high has poor fluidity. Therefore, when the electrolytic solution in the cell has a portion having high gas/liquid ratio, the electrolytic solution has poor circulation, so that not only is the concentration of the electrolytic solution locally lowered but also the gas is likely to be stagnated in the electrolytic cell. The ratio of a portion of the electrolytic solution having high gas/liquid ratio can be decreased to some extent by a method in which the electrolysis pressure is increased or the amount of the electrolytic solution circulated is greatly increased. However, such a method for decreasing the ratio of a portion of the electrolytic solution having high gas/liquid ratio poses problems in that safety is sacrificed and the cost for equipment becomes high.
Conventionally, many proposals have been made with respect to the unit cell for the ion exchange membrane electrolysis of an alkali metal chloride, in which a high purity alkali metal hydroxide can be produced at high current density. For example, these proposals are made in Unexamined Japanese Patent Application Laid-Open Specification No. 51-43377 (corresponding to U.S. Pat. No. 4,111,779), Unexamined Japanese Patent Application Laid-Open Specification No. 62-96688 (corresponding to U.S. Pat. No. 4,734,180), and Japanese Patent Application prior-to-examination Publication (kohyo) No. 62-500669 (corresponding to U.S. Pat. No. 4,602,984). The unit cells disclosed in these patent documents have a defect in that, in the operation of these unit cells, the withdrawal of a gas and a liquid from an upper portion of the cells is performed in a condition wherein the gas and the liquid get mixed with each other, so that vibrations occur in the cell and the vibrations cause a breakage of an ion exchange membrane. Further, these unit cells are not adapted for facilitating the circulation of the electrolytic solution therein. Therefore, for rendering narrow the concentration distribution of the electrolytic solution in the cells, it is necessary to circulate a large amount of an electrolytic solution.
Unexamined Japanese Patent Application Laid-Open Specification No. 61-19789 and U.S. Pat. No. 4,295,953 disclose a unit cell in which a cell frame has a hollow structure and is of a picture frame-like shape, and an electrically conductive distributor is disposed between an electrode plate and an electrode sheet, wherein the distributor is intended to serve as a path for the downward flow of an electrolytic solution. Unexamined Japanese Patent Application Laid-Open Specification No. 63-11686 discloses a unit cell in which a cell frame has a hollow structure and is of a picture frame-like shape, and a cylindrical member for electrical current distribution is provided, wherein the cylindrical member is intended to serve as a path for the downward flow of an electrolytic solution. In these prior art techniques, an improved circulation of an electrolytic solution in the cells can be attained, but when the electrolysis is conducted at a high current density, it is likely that vibrations occur around an outlet for a gas and liquid and that a gas is stagnated in the upper portion of the cells. Further, these techniques have a problem in that the cells have a complicated structure. Unexamined Japanese Utility Model Application Laid-Open Specification No. 59-153376 proposes a method for preventing the occurrence of vibrations in an electrolytic cell, which comprises disposing a mesh body for preventing bubble growth in the upper portion (near the liquid surface of the electrolytic solution) of the electrode compartment. However, by this method, a gas-liquid separation cannot be satisfactorily performed, so that this method cannot completely prevent the occurrence of vibrations due to the pressure fluctuations in the electrolytic cell.
Unexamined Japanese Patent Application Laid-Open Specification No. 4-289184 (corresponding to U.S. Pat. No. 5,225,060) discloses an electrolytic cell employing a unit cell which contains anode-side and cathode-side gas-liquid separation chambers respectively disposed in anode-side and cathode-side non-current-flowing spaces and extending over the entire lengths of the upper sides of the anode and cathode compartments, wherein each of the gas-liquid separation chambers has a gas and liquid outlet nozzle, which opens downwardly so that a gas and liquid having been separated from each other by the gas-liquid separation chamber can be discharged while maintaining the gas-liquid separated state. Further, the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 4-289184 also teaches a method in which an L-shaped duct is disposed in at least one of the anode compartment and the cathode compartment, wherein the duct is intended to promote the circulation of an electrolytic solution in the electrode compartment. In the case of the use of the above-mentioned electrolytic cell, when the electrolysis is performed at a current density of 45A/dm2 or less, advantages can be obtained in that the occurrence of vibrations is relatively small and the concentration distribution of an electrolytic solution in the electrode compartment is narrow. However, when the electrolysis is performed, for example, at a current density as high as 50 A/dm2 or more by using the above-mentioned electrolytic cell, an extremely great amount of gas bubbles are formed in the electrolytic cell. As a result, a satisfactory gas-liquid separation cannot be effected, so that problems are posed in that great vibrations are caused to occur, thus adversely affecting an ion exchange membrane, and the concentration distribution of the electrolytic solution becomes broad.
Unexamined Japanese Patent Application Laid-Open Specification No. 8-100286 (corresponding to U.S. Pat. No. 5,571,390) proposes that a number of vertically extending ducts (downcomers) are disposed in the electrode compartments of a unit cell which contains gas-liquid separation chambers, such as the unit cell as already described above. However, even the unit cell (containing downcomers) proposed in this patent document poses a problem in that, when the electrolysis is performed at a current density as high as 5 A/dm2 or more, the gas-liquid separation becomes unsatisfactory and, hence, great vibrations are caused to occur, thus adversely affecting an ion exchange membrane.
In this situation, the present inventors have made extensive and intensive studies with a view toward developing a unit cell for use in a bipolar, filter press type electrolytic cell used for performing the ion exchange membrane electrolysis, wherein the unit cell is advantageous in that a gas and an electrolytic solution can be discharged in a condition wherein the gas and the electrolytic solution are substantially completely separated from each other, so that, even when the electrolysis is performed at a current density as high as 50 A/dm2 or more, the occurrence of vibrations in the cell can be prevented, thereby preventing the occurrence of a breakage of an ion exchange membrane. As a result, it has surprisingly been found that discharge of a gas and a liquid in a substantially completely gas-liquid separated condition can be achieved when the electrolysis of an aqueous alkali metal chloride solution is performed by using a bipolar, filter press type electrolytic cell which employs a unit cell comprising: an anode-side pan-shaped body having an anode compartment and an anode-side gas-liquid separation chamber which extends over the entire length of the upper side of the anode compartment, and a cathode-side pan-shaped body having a cathode compartment and a cathode-side gas-liquid separation chamber which extends over the entire length of the upper side of the cathode compartment, wherein the anode-side pan-shaped body and the cathode-side pan-shaped body are disposed back to back, wherein the anode-side and cathode-side gas-liquid separation chambers have perforated bottom walls separating the anode-side and cathode-side gas-liquid separation chambers from the anode compartment and the cathode compartment, respectively, wherein a bubble removing partition wall having an apertured segment is disposed at least in the anode-side gas-liquid separation chamber of the anode-side and cathode-side gas-liquid separation chambers and extends upwardly of the perforated bottom wall of the gas liquid separation chamber, wherein the bubble removing partition wall extends along the entire length of the gas-liquid separation chamber to partition the gas-liquid separation chamber into a first passage A formed on the bottom wall in a perforated area thereof and a second passage B which is formed on the bottom wall in a non-perforated area thereof and which communicates with a gas and liquid outlet nozzle, and wherein the apertures of the apertured segment of the bubble removing partition wall are positioned at least 10 mm above the inside surface of the bottom wall of the gas-liquid separation chamber. The present invention has been completed, based on this novel finding.
Accordingly, it is an object of the present invention to provide a unit cell for use in a bipolar, filter press type electrolytic cell, wherein the unit cell is advantageous in that a gas and an electrolytic solution can be discharged in a condition wherein the gas and the electrolytic solution are substantially completely separated from each other, so that, even when the electrolysis is performed at a current density as high as 50 A/dm2 or more, the occurrence of vibrations in the cell can be prevented, thereby preventing the occurrence of a breakage of an ion exchange membrane.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.